No fish left behind
Strategies to get more bang for your feed buck
By Ron Hill
The core of commercial aquaculture is feeding. Feeding and growing your stock is where money is made, and feed is one of the biggest expenses at any farm.
There is a wide range of feeding strategies across the industry, and techniques vary even between farms growing the same species. Indeed, it is often said that no two farm managers feed the same way. Successful farmers use the same principles, even if the individual techniques vary, to get the best growth.
Getting the best growth and largest biomass is dependant on the ability of the farmer to deliver feed to each fish, making sure the maximum number of fish gets access to feed. Getting feed to fish in a tank or cage seems like a simple task but fish being left behind, uneven growth and too many undersized fish at harvest are common at many struggling farms.
Many technologies and sophisticated strategies costing thousands of dollars exist to help farmers get the best feed distribution, but these must be employed strategically and correctly to increase growth and be a boon to the farmer. Understanding fish populations and fish behaviour in the tank or pen is the first step to minimizing uneven growth and maximizing feed efficiency and biomass.
Fish in any tank or pen will set themselves up in a feeding hierarchy. The biggest and strongest fish will take position to give themselves the best access to feed and will exclude the smaller fish, forcing them to the margins where feed access is more difficult.
The farmer must outflank the feeding hierarchy to get the best biomass growth out of the stock by ensuring the lowest fish in the hierarchy have access to feed and growth. If feed is too concentrated in one area the fish must compete harder for access and can injure each other as they collide, losing their slime coat and scales.
Good feed distribution, by spreading the feed out across the whole tank or pen, will allow more fish access to feed allowing better biomass growth. Fish who are “left behind” start as fish that access less feed and therefore, grow much slower than the rest of the population. Slow growth increases their competitive disadvantage to access feed as the other fish grow. Eventually, the size difference between these marginalized fish and the average fish becomes so great that the marginalized fish are too small to keep up with the average-sized fish in the tank.
Having no fish left behind is close to impossible but minimizing these fish and maximizing biomass growth across the population is a huge financial concern. In large operations, fish left behind can be thousands of fish potentially worth thousands of dollars.
Coefficient of variation
Farmers use the term CV (co-efficient of variation) to describe the size variation found within a population of fish. The smaller the CV number, the less variation occurs in the population. The larger the CV, the larger the variation in the population size – more fish of different sizes.
CV = standard deviation / average weight
The CV calculation yields a number that farmers compare to their own current and historical data, or those of similar operations. Because of the diverse nature of aquaculture, fish species and rearing methods, apples to apples CV comparisons are the most valuable to the farmer.
The farmer needs a data set of individual weight measurements to calculate the standard deviation, thus, CV is infrequently calculated in real time. Farmers will talk about CV in general terms day-to-day based on observation: “The CV in this tank looks high.” A low CV in a tank or pen indicates good feeding practices as most of the fish are growing at the same rate and are accessing feed.
Strategizing in 3D
Technology is a huge factor in feeding and ensuring proper feed distribution. Many farmers without the benefits of modern equipment feed based on what their eyes tell them – judging hunger and satiation on surface reaction.
Feed blowers, spinners and modern feed systems are excellent tools for distributing feed across the surface of the pen or tank, but the farmer must still consider the three-dimensional nature of the tank or pen.
It is too easy to assume that fish will simply rotate through the surface to get food and drop down once they are satiated. Even if fish are circling in a cage or tank, there is still a feeding dominance hierarchy. Dominant fish aren’t necessarily getting out of the way to let other fish feed, even if they are satiated. Some fish that are lower in the water column will come up to access feed but they must push their way through and fight for space and feed. Some fish stay below the surface and take what comes to them.
Feeding is often based on surface feeding and daily ration calculations. However, the advent of underwater cameras (and the use of viewing windows) gives farmers the ability to observe fish eating below the visible surface and to ensure that feed is not being wasted.
Frequent camera-based feeding allows fish to become accustomed to feeding within the water column as they realize they don’t need to compete at the surface for feed, since farmers don’t stop feeding once the surface reaction has diminished and feed can be given at a faster rate without waste.
Ensuring a widespread feed distribution, grading and a strategic feeding regime to outflank the feeding hierarchy will get the best out of the tank. Handfeeding to complement automated feeders allow the technician to supplement the feeder and provide feed to spots in the tank that receive less feed, “top up” any hungry fish at the surface or at depth, and to judge the amount of feed being dispensed by the automated feeder.
Whatever techniques are used to get the best feed spread, attention to detail and knowledge of fish behaviour are the best allies to get feed to the most fish.